An instrument for performing lunar distance calculations for navigation, designed by M. Hue, Professor of Hydrogaphy. The plotter's scales are printed, with brass attachments for reading the scales and performing calcuations.

Base: Instructions and method of use are printed on lower part of base. A card of tables is carried in a sleeve on the underside of the base. A brass rule carrying a magnifying glass with cross wires is mounted across face of base, fitted with a brass clamp at each end operating on edges of board.

Rotating disc: 13.6" (34.5 cm) diameter. Cardboard disc mounted at centre of base. Graduated anticlockwise 0 to 180 degrees and quadrantally in degrees by 30'. Fitted with two brass vernier scales 0 - 30 to 1' attached to base 180 deg apart, inscribed "Divisiovs positives" "Divisions negatives"

The text printed around the main planisphere roughly translates:
'The resolution of spherical triangles by graphical procedures, proposed initially by the hydrographic engineer Keller (Double Planisphere).
M. Hue, professor of Hydrography, had the idea of combining the modifications brought to the construction of the planisphere of M.M Keller by M M Saxby and Zescevich to obtain an instrument specially intended for lunar distances.
The well-understood arrangement of this instrument and the tables which are attached ...M. Hue permitted in effect to carry out the reduction of distances with great speed and an approximation which would be sufficient in practice. (Depot de la Marine)

INSTRUCTIONS FOR LUNAR DISTANCE
The apparent altitudes of the moon and the other star one deduces to view their apparent height: one calculates exactly the horizontal parallax of the moon and one corrects the semi – diameter the observed distance to conclude the apparent distances of the centres. The distance given right, one lifts the rule (below the horizontal diameter just crossing the cursor corresponding to the apparent height of the moon, one turns the disc to a quantity equal to the distance and one pushes the cursor on the parallel of the apparent height of the other body/star . One notes the distance +/- of the cursor at the axis. Replacing the right disc one notes also the distance +/-of the cursor at the axis.
One takes the difference between the corrected refraction (Table 1) which agrees with the height of the second star and its parallax, if it is not zero, then one takes the (Table II ) with the minutes and seconds of the corrected refraction, neglecting the numeral of the ten or so of the horizontal argument.

This number which, neglecting the thousands, multiplied by N’ gives the correction relative to the second star, that which has the same sign as N’. After that, one (retraces ?) of the horizontal parallax of the moon the corrected refraction (Table I) taken through the apparent height and one reached (Table II) the number of the (thousands) corresponding to this difference. The product of this number N, adjusted next with N, gives the corrections due to the moon, correction of the same sign N.

Finally, having taken (Table III) the parallax in the height of the moon, one seeks in Table IV two numbers corresponding, the one at the apparent distance and the parallax in height the other at the same distance and the combination of the two prime corrections, their difference, positive if the distance is less than 90 degrees, negative in the contrary case, gives the complimentary correction.
The algebraic sum of the three corrections, applied to the apparent distance, makes known the true distance'.